The present invention relates to nozzle segments for gas turbines and particularly relates to steam cooled gas turbines having nozzle covers spaced from the nozzle wall defining the hot gas path and pedestals within the nozzle segments for interconnecting the nozzle wall and cover to reduce pressure-induced stress.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner bands and a plurality of vanes extend between the bands. The bands and vanes define in part the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more nozzle vanes extend between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover spaced radially from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. Each impingement plate defines with the cover a first cavity on one side thereof for receiving cooling steam from a cooling steam inlet and also defines along an opposite side thereof, and with the nozzle wall, a second cavity. Each impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the associated nozzle wall. The cooling steam from the second cavity of the outer band portion flows radially inwardly through cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. The cooling steam then enters a radially innermost first cavity in the inner band portion and reverses its flow direction for flow radially outwardly through an impingement plate into the associated second cavity for impingement cooling the nozzle wall of the inner band. The spent cooling medium returns through a cavity in the vane to an exhaust port of the nozzle segment radially outwardly of the outer band portion.
The cover provided each of the outer and inner band portions is preferably welded to the corresponding nozzle segment wall about the lateral margins of the nozzle segment, i.e., the leading and trailing edges and side edges of the segment. Consequently, a closed cooling system is provided through the nozzle segment in which the cooling medium, e.g., steam under pressure, flows through the band portions and the vanes. The steam, however, is contained within the chambers at different pressure levels as compared with the pressure of the gas path and the compressor discharge flow into portions of the fixed turbine casing surrounding the outer band portion. This pressure difference can cause high stress for the nozzle segment, especially at the joint region between the cover and the nozzle wall. The stress tends to balloon the cover and nozzle wall away from one another, bending the welded joint along the margins of the cover and nozzle wall.
These pressure-induced stress levels cause local high stress at the joint and the fillets interconnecting the nozzle wall and the vane. These high local stresses can reduce the low cycle fatigue life of the part. While a thicker wall or enhanced cooling scheme can be employed to cure some of these problems, each of those methods has serious drawbacks. For example, a thicker wall causes a high thermal gradient which has an adverse effect on the low cycle fatigue life of the part. Enhanced cooling is not always available and can be expensive in terms of turbine performance.
In accordance with a preferred embodiment of the present invention, one or more structural elements, for example, pedestals, are interconnected between the cover and the nozzle wall to structurally support these nozzle parts and reduce the stress induced by the pressure differences in the closed loop cooling system of the turbine. By reducing those stresses, low cycle fatigue at the previously localized highly stressed parts is increased. To accomplish the foregoing, one or more structural pedestals are provided, interconnecting the cover and nozzle wall at locations within the chamber defined between the cover and the nozzle wall. The pedestals are spaced from the lateral margins of the nozzle segment and are located at one or more areas to preclude substantial ballooning of the nozzle segment wall and cover away from one another responsive to internal and external pressure differences. The pedestals are preferably in the form of pins which can have suitable cross-sections, such as circular, multi-sided or elongated. The pins are preferably cast with the nozzle wall and vane in a single crystal casting with the distal ends of the pedestals received through openings in the impingement plates and in openings in the covers. The distal ends are welded to the covers by a TIG welding or E-beam welding process externally of the segment. Alternatively, the pedestals can be cast on the cover and welded to the nozzle band or comprise discrete pedestals welded at both ends to the nozzle wall and cover. Preferably, the pedestals are located on each of the opposite sides of the opening of the vane through the nozzle wall, i.e., between the nozzle vane openings and the lateral margins of the segment.
In a preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall, the cover and the nozzle wall being secured to one another about margins thereof and defining a chamber therebetween and at least one structural element interconnecting the cover and the nozzle wall inwardly of the margins to substantially prevent movement of the cover and the nozzle wall relative to one another in a generally radial direction.
In a further preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall, the cover and the nozzle wall being secured to one another about margins thereof and defining a chamber therebetween, an impingement plate secured within the segment and disposed in the chamber to define with the cover a first cavity on one side thereof for receiving a cooling medium, the impingement plate on an opposite side thereof from the first cavity defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures therethrough for flowing the cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, and at least one structural element interconnecting the cover and the nozzle wall inwardly of the margins to substantially prevent movement of the cover and the nozzle wall relative to one another in a generally radial direction, the impingement plate including a hole therethrough for receiving the structural element, the element and the hole lying laterally outwardly of a juncture between the vane and the nozzle wall.